1. Field of the Disclosure
The present invention relates generally to power converters, and more specifically, the invention relates to improving output voltage regulation at light/no load conditions.
2. Background
Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. are powered by a source of dc power. Because power is generally delivered through a wall outlet as high-voltage ac power, a device, such as a power converter, is required to transform the high-voltage ac power to usable dc power for many electrical devices. In operation, a power converter may use a controller to regulate energy delivered to an electrical device that may be generally referred to as a load. In one instance a controller may control the transfer of energy pulses by switching a power switch on and off in response to feedback information of an output voltage to keep the output voltage at the output of the power converter regulated.
In certain applications, power converters may include an energy transfer element to separate an input side from an output side of the power converter. More specifically, the energy transfer element may provide galvanic isolation which prevents dc current from flowing between the input and the output of the power converter and may be required due to certain safety regulations. A common example of an energy transfer element is a coupled inductor, where electrical energy received by an input winding on the input side is stored as magnetic energy and then converted back to electrical energy at the output side of the power converter across an output winding.
For certain power converter designs it may be required to regulate the output voltage within a specified voltage range. To accomplish this, some power converters may use ‘primary feedback’ to allow indirect sensing of the output voltage from the input side of the power converter in order to regulate an output voltage within a specified range. Primary feedback may be used instead of circuitry that directly senses an output voltage at the output of the power converter in order to reduce costs. One example of primary feedback is to electrically couple a bias winding to the input side of the power converter such that it is also magnetically coupled (not directly connected) to the output winding of the energy transfer element. This allows the bias winding to produce a voltage representative of the output voltage of the power converter across the bias winding, which shares an electrical connection with the input side of the power converter. In this manner, the power converter acquires a feedback signal representative of the output voltage without directly sensing the output voltage at the output of the power converter.
However, when implementing a primary feedback for regulation in a power converter at substantially light/no load conditions (where the load demands very little or no power), the output voltage may deviate substantially from its desired value. This may prevent the output voltage from being within the specified output voltage range at light/no load conditions.